1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, printer, facsimile, or the like using an electrophotographic system.
2. Description of the Related Art
As a prior art relating to a hybrid development method (hereinafter, referred to as an HV development method), an image forming apparatus described in Japanese Patent Laid-Open No. H9-211970 is known. Japanese Patent Laid-Open No. H9-211970 discloses an image forming apparatus including a developing roller which carries a toner facing a photosensitive drum, and a conveying roller which carries a two-component developer including the toner and a magnetic carrier facing the developing roller. In the image forming apparatus, electric fields are made to act between the developing roller and the conveying roller to form a toner layer on a surface of the developing roller, and develop an electrostatic image of the photosensitive drum.
In the HV development method, since the charging of the toner is performed by stirring the two-component developer, a sufficient charging amount may be easily obtained, and since the supply of the toner from the conveying roller to the developing roller is performed by an electrostatic force, a toner charged to opposite-polarity is not supplied to the developing roller. Therefore, an occurrence of fog may be prevented by avoiding a toner adhesion to a non-image area of a photosensitive drum 1. Further, since only the toner is supplied to the developing roller, there are advantages such as adhesion of the magnetic carrier to the photosensitive drum 1 also being prevented or the like.
FIG. 1 is a schematic view illustrating a development device 20 (hereinafter, referred to as an HV development device) having a configuration of Japanese Patent Laid-Open No. H9-211970 employing the HV development method. The two-component developer in a developing container 21 is supplied to a developer carrier 31 having a magnet fixedly disposed therein by a supply member 30. The supplied two-component developer is conveyed to a facing portion with a toner carrying member 27, while being controlled by a limiting member 32.
A potential difference ΔV is applied to the facing portion by a voltage applying portion 26. The toner of the developer in the facing portion is separated from the magnetic carrier on which the toner is electrostatically adhered by the ΔV, and projected in a direction of the toner carrying member 27 so as to be coated thereon. In this case, the ΔV and a charge amount Q/S in a unit area of the toner to be coated are in a proportional relationship as shown in Equation 1.[Equation 1]ΔV∝Q/S=M/S×Q/M  (1)
Wherein, Q/S (μC/cm2) is a product of a toner amount M/S (g/cm2) in the unit area and the charge amount Q/M (μC/g) in a unit mass of the toner.
The toner coated on the toner carrying member 27 is conveyed to the facing portion with the photosensitive drum 1 to develop the electrostatic image on the photosensitive drum 1.
Meanwhile, in order to reduce energy consumption, a development device capable of outputting a high-quality image with a small toner amount is required. Therefore, speaking of the toner, by increasing an amount of pigment contained in the toner or improving dispersibility of the pigment, attempts to improve a density per toner have been made. However, in the HV development device, although the toner with improved density is used, it can be seen that an effect of suppressing the toner amount is limited.
FIG. 2A is a schematic view illustrating a toner (particle diameter=7.6 μm, specific gravity=1.1 g/cm3, and M/S=0.47 mg/cm2) developed on the photosensitive drum 1 by the HV development device. FIG. 2B is a schematic view when the toner is developed with a high density on the surface of the photosensitive drum 1 with the same toner amount.
As compared to a toner image (FIG. 2B) with a high density of the toner occupying the surface of the photosensitive drum 1, a toner image (FIG. 2A) with a low density is partially exposed due to the toner not completely covering the surface of the photosensitive drum 1 with the same toner amount. Therefore, when the toner image is transferred onto a sheet, due to the influence of a white background portion where the toner is not present, the image density is significantly reduced. In addition, it can be seen that density unevenness between a part having a large toner amount and a part having an extremely small toner amount is noticeably increased.
FIG. 2C is a schematic view illustrating a toner image (particle diameter=7.6 μm, specific gravity=1.1 g/cm3, and M/S=0.65 mg/cm2) when the image density is improved by increasing the potential difference ΔV of the HV development device. As illustrated in FIG. 2C, it can be seen that, in order to improve the image density, a much greater amount of toner than necessary is developed, and it is necessary to coat the surface of the photosensitive drum 1, and thus the effect of suppressing the toner amount is limited.
FIG. 3 is a graph illustrating results of a density of toner on media after fixing by an oven relative to a toner amount M/S (mg/cm2) on the same media. The media used are Intelimer sheets (manufactured by Nitta Corporation) turnable on/off the adhesive force depending on a temperature condition.
A graph a of FIG. 3 is results in which the adhesive force of the Intelimer sheet is turned off depending on the temperature condition, and the toner image is fixed on the media by outputting a normal image by an image forming apparatus having the HV development device.
Meanwhile, a graph b of FIG. 3 is results in which the adhesive force of the Intelimer sheet is turned on depending on the temperature condition, and a high-density toner image as illustrated in FIG. 2B is achieved and fixed on the media by spreading the toner on the media and removing an excess toner by air. The HV development device does not reach a saturation density unless a large toner amount is developed to cover the surface of the photosensitive drum 1, whereas, if the high-density toner image is implemented, it is possible to cover the surface of the photosensitive drum 1 with a small toner amount and still reach a saturation density.
As described above, it is difficult to obtain a desired density with a small toner amount by using the HV development device and improve the density unevenness. Thereby, the present inventors examine the cause of a decrease in the density of the toner image developed on the photosensitive drum 1 in the HV development method. As a result, it can be seen that, in a method of coating the toner covered on the magnetic carrier by using the potential difference between both rollers as in the HV development device, the density of the toner image is easy to be reduced mainly by the following two reasons.
(1) When coating the toner on the surface of the toner carrying member 27 by the potential difference between the developer carrier 31 and the toner carrying member 27 illustrated in FIG. 1, since a force acts on the toner present in a space to which the electric fields are applied, such that the toner has multiple forces acting thereon, it is difficult to uniformly dispose the toner on the surface. In addition, the toner is multi-layered on the surface, such that the density of toner occupying the surface of the toner carrying member 27 is easy to be reduced as illustrated in FIG. 2A.
(2) Further, when the toner carried on the toner carrying member 27 is projected to the photosensitive drum 1, in the case of the toner being formed in a multi-layered non-uniform toner layer as illustrated in FIGS. 2A and 2C, since the adhered amount of the toner is different from each other, a development residue is easy to be generated, and the density of the toner image developed on the photosensitive drum 1 may be further reduced.